The invention relates to a centrifuge comprising a rotating drum and comprising an ejection element which rotates together with the drum in order to eject a cake of solids which is deposited on the inner side of the drum in the axial direction of the drum.
The development of centrifuges divided up very early into different special applications in order to meet the differing needs of the process technology. The filter cake of a sedimentation or filtration centrifuge can be a mayonnaise-like mass, a rheologically pasty mass or a mass in the form of a saturated or unsaturated agglomeration with a Mohr breaking limit curve.
Thus there is for example an inverting filter centrifuge by the company Heinkel GmbH, D-74303 Bietigheim-Bissingen, Germany, in which a charge-wise filling, washing, spin drying and emptying by means of a stroke over the entire drum length are possible. Movable cloth is used as an inverting filter, which is accompanied by a not inconsiderable risk that cloth abrasions enter into the product. A quasi continuously operating double pusher centrifuge is show in patent specification EP 0 635 309 in which an intermittent pusher floor is arranged in a sieve drum and alternately moves a ring of solid material which has arisen in its xe2x80x9cwakexe2x80x9d axially outwardly by a stroke length. A disadvantage of the pusher centrifuges consists in that in freshly deposited filter cakes the maximum arising pusher pressure which occurs over a saturated agglomeration is the greatest at the pusher floor.
It is an object of the present invention to show a centrifuge which can be adapted to different products and can permit different operating modes. This object is satisfied in that the ejector element has an ejector surface in the form of a screw-shaped helix, and in that the helix and the drum are kinematically connected in such a manner that a reference point at the periphery of the helix executes a rotational movement relative to the drum in the shape of a sawtooth line, with a first flank of the sawteeth corresponding approximately in its inclination to the pitch of the helix whereas a second flank of the sawteeth corresponds to an approximately axial ejection movement.
This arrangement has the advantage that by means of a small but repeated stroke in the axial direction, a reliable ejection of the solid cake which is largely independent of the product properties takes place due to the axial extent of the helix over the entire length of the drum. At the same time constructional advantages arise since only a small stroke is required for the ejection movement in the axial direction. Because the backward movement of the helix in its own path takes place through a combination of rotation and translation relative to the drum, with it being possible to select the speeds during the passing through of the first and second flanks, that is, the backward rotational and feed movement, a simple adaptation to different properties of the solid cake is possible.
If the inclination of the first flank of a sawtooth agrees exactly with the pitch of the helix, the lowest resistances arise to the rotating back of the helix in its own path. This can be exploited through an intentional choice of the absolute direction of rotation of the drum and the helix in regard to the pitch of the helix in such a manner that the guiding of the solid cake is sufficient in order for example to rotate back a multiple thread helix in its path with a braking torque with respect to the rotation of the helix and with an axial draw force. An apparatus of this kind would for example require only a hydraulic displacing piston and a load switching coupling with a brake instead of a second drive for the helix. The kinematic connection for the backward running in its own path would then be given through the helix in the solid cake itself.
If the relative rotational movement between the helix and the drum is intentionally controlled in such a manner that the first flank of the sawteeth deviates somewhat from the pitch of the helix, a trench can arise at the helix during the passing through of this flank, which can be very helpful in order additionally to collect and drain off liquid. The latter can emerge axially out of the cake or stand above the cake. If the inclination of the first flank is less than the pitch of the helix, a slight ejection movement also still arises while passing through the first flank, and at the other side of the helix a trench arises if the solid cake is only locally yielding in slight ejection movements.
If the relative movement on the sawtooth line is for example hydraulically controlled and a reversal of the movements on the sawtooth line is possible, then a plurality of process stages can also be performed charge-wise. Thus for example when viewed in the axial direction a suspension can first be filled in the middle of the drum and at first be centrifuged in order to give off liquid, and then be brought into a washing region of the drum through xe2x80x9cbackward running on the sawtooth linexe2x80x9d and, after the washing and compressing of the solids, transported in the ejection direction over the filling-in position through a xe2x80x9cforward running on the sawtooth linexe2x80x9d. A charge-wise operation also permits a superimposed drying such as for example vapor pressure or compressed air dehumidification.
A further possibility which can be relatively simply realized with a hydraulic piston via a valve control consists in superimposing a vibration in the axial direction on the helical movement in order to exploit the rheological properties of the cakes for the further dewatering.
Simple control systems for centrifuges of this kind can also be such that a continuous, slow rotational movement is produced between the helix and the drum. The axial movement of the helix is produced by a hydraulic piston, which has a velocity component in the axial direction corresponding to the tangential velocity of the helix in order to form a suitably composed velocity in the direction of the first flank of a predetermined sawtooth. If a slower rotational movement between the helix and the drum is chosen, then the time interval for the deposition of the cake increases. For the second flank of the sawtooth a rapid stroke takes place in the opposite direction in order to move the solid cake axially in the ejection direction.
The invention will be described in the following with reference to exemplary embodiments.